Method for Producing Hot Rolled Strip with a Multiphase Microstructure

ABSTRACT

For the production of hot strip referred to as TRIP steel (transformation induced plasticity), with a multiphase structure and with outstandingly good deformation properties along with high strengths, from the hot-rolled state, the invention proposes a method which is carried out with a predetermined chemical composition of the steel grade used within the limits 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements and with a combined rolling and cooling strategy in such a way that a structure comprising 40-70% ferrite, 15-45% bainite and 5-20% residual austenite is obtained, wherein the finish rolling of the hot strip ( 7 ) is performed to set a very fine austenite grain (d&lt;8 μm) in the final forming operation (6′) at temperatures between 770 and 830° C. just above Ar3 in the region of the metastable austenite, and a controlled two-stage cooling ( 10, 11, 12 ) is carried out after the last rolling stand ( 6′ ) of the hot strip ( 7 ) to a strip temperature in the range of bainite formation of 320-480° C., with a holding time of about 650-730° C., the beginning of which is determined by the entry of the cooling curve into the ferrite region and the duration of which is determined by the transformation of the austenite into at least 40% ferrite.

The invention concerns a method for producing hot-rolled strip that consists of TRIP (transformation-induced plasticity) steel with a multiphase microstructure and with both high strength values and outstanding deformation properties, where the TRIP steel strip is produced from the hot-rolled state by controlled cooling after the last rolling stand.

The adjustment of the microstructure is a complex matter in TRIP steels, since, besides ferrite and bainite, a third phase is present in the form of retained austenite or, after a subsequent deformation, in the form of martensite. TRIP steels are now usually produced in a two-stage heat cycle. The starting material is hot-rolled or cold-rolled strip, in which an approximately 50% α-50% γ initial microstructure is adjusted. Due to the higher solubility of carbon in austenite, austenite has a higher carbon concentration. After the annealing treatment, rapid cooling is carried out past the ferrite and pearlite range into the bainite range, in which isothermal conditions are maintained for some time. The austenite is partially transformed to bainite, and at the same time the remainder of the austenite becomes further enriched with carbon. In this way, the martensite start temperature M_(s) is reduced to values below ambient temperature, and consequently the retained austenite also continues to exist at ambient temperature. The final microstructure consists of 40-70% ferrite, 15-40% bainite, and 5-20% retained austenite.

The special effect of TRIP steels is the transformation of the metastable retained austenite to martensite when external plastic deformation occurs. The transformation of the austenite to martensite is accompanied by an increase in volume, which is supported not just by the austenite alone but rather by the surrounding microstructural components as well. The ferritic matrix is plasticized, which in turn results in greater strain hardening and leads overall to higher plastic elongations. Steels produced in this way have an extraordinary combination of high strength and high ductility, which makes them suitable especially for use in the automobile industry.

The process management described above which is presently used mostly for the industrial production of TRIP steels, is complicated and expensive due to the additional annealing and cooling treatment after the rolling operation, which is the reason that attempts have been made to produce these TRIP steels directly as hot-rolled strip in industrial production plants for hot strip production. For example, EP 1 396 549 A1 discloses a method for producing pearlite-free hot-rolled steel strip with TRIP properties in a continuously running operational process, in which a steel melt, which contains, in addition to iron and unavoidable impurities, 0.06-0.3% C; 0.1-3.0% Si; 0.3-1.1% Mn (with the total amount of Si and Mn being 1.5-3.5%); 0.005-0.15% of at least one of the elements Ti or Nb as an essential component; and optionally one or more of the following elements: max. 0.8% Cr; max. 0.8% Cu; and max. 1.0% Ni, is cast into thin slabs, which are annealed at 1,000-1,200° C. for an annealing time of 10-60 minutes in an annealing furnace, starting from a run-in temperature of 850-1,050° C. After descaling, the thin slabs are finish hot rolled in the range of 750-1,000° C. and then cooled to a coiling temperature of 300-530° C. The controlled cooling is carried out in two stages at a cooling rate of the first stage of at least 150 K/s with a cooling interruption of 4-8 seconds. Alternatively, it is proposed that the controlled cooling be carried out continuously at a cooling rate of 10-70 K/s without a holding interruption. Finally, a third possibility is proposed, in which the cooling is controlled in such a way that the hot rolled strip is cooled in a first phase to a temperature of about 80° C. above coiling temperature within 1-7 seconds and is then cooled to coiling temperature by air cooling. Besides the prescribed process management, the presence of Ti and/or Nb is important, since these elements remain in solution until the start of the hot rolling and, upon their subsequent precipitation, improve, among other properties, the grain fineness of the hot-rolled strip and increase the retained austenite content and its stability.

Using this prior art as a point of departure, the objective of the invention is to specify a method which allows simpler and more economical production of TRIP steels in existing plants and in which an annealing treatment and the addition of alloying elements that are otherwise not absolutely necessary can be eliminated.

This objective is achieved by the characterizing features of claim 1, according to which the production of the hot-rolled strip in a thin-slab continuous casting and rolling plant (CSP plant) with a predetermined chemical composition of the steel grade that is used within the following limits: 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements is carried out with a combined rolling and cooling strategy in such a way that a microstructure is obtained which consists of 40-70% ferrite, 15-45% bainite, and 5-20% retained austenite, such that

-   -   the finish rolling of the hot-rolled strip for adjusting a very         fine austenite grain (d<8 μm) during the last deformation is         carried out at temperatures of 770-830° C., just above Ar₃ in         the range of metastable austenite, and     -   immediately after the last rolling stand, a controlled two-stage         cooling of the hot-rolled strip to a strip temperature in the         range of bainite formation of 320-480° C. is carried out with a         holding time at about 650-730° C., whose start is determined by         the entry of the cooling curve into the ferrite range and whose         duration is determined by the transformation of the austenite to         at least 40% ferrite.

In contrast to the usual procedure that was described earlier, in accordance with the invention, in an austenitically finish rolled hot strip, the typical microstructure for a TRIP steel is adjusted immediately after the last rolling stand by a two-stage cooling operation in the cooling line. In this connection, the adjustment of the appropriate microstructure requires extensive process know-how as well as very exact maintenance of the necessary process parameters. Due to the narrow tolerance range for the production of TRIP steels on hot wide strip mills, since the introduction of thin-slab continuous casting and rolling technology, a plant configuration has been available which provides much better conditions for the direct production of TRIP steels than hot-rolled strip, compared to conventional hot-rolled strip mills. Due to the high degree of uniformity of temperature over the thickness, width, and length of the strip, TRIP steels with constant mechanical properties can be reproducibly produced in this way. Due to the short length of the conventional cooling lines used in this process in existing continuous casting and rolling mills, the production of hot-rolled strip with TRIP microstructure is possible only with a special rolling and cooling strategy.

The rolling strategy of the invention is used for adjusting a very fine austenite grain (d<8 μm) during the last deformation, which has an accelerating effect on the ferrite transformation in the subsequent cooling line. Therefore, the finish rolling of the strip takes place at temperatures of 770-830° C., just above Ar₃ in the range of metastable austenite.

The successful implementation of the cooling strategy makes it absolutely necessary to maintain certain limits of chemical composition in order to realize the desired degree of transformation within the short total cooling time that is available. Therefore, the chemical analysis proposed for the production of TRIP steels varies within the following limits: 0.12-0.25% C, 0.05-1.8% Si, 1.0-2.0% Mn, the remainder Fe and customary accompanying elements.

The cooling strategy involves two-stage cooling with the option of using different cooling rates in each stage. The start of the holding time at temperatures of 650-730° C. is determined by the entry of the cooling curve into the ferrite range. The desired transformation of the austenite to at least 40% ferrite then takes place during the following brief cooling interruption. The holding time is then immediately followed by the second cooling stage, in which the hot-rolled strip is cooled to a temperature of 320-480° C. The transformation of austenite to at least 15% bainite takes place at this temperature.

In addition to the use of a short holding time, the cooling strategy is determined by an exactly defined, predetermined cooling rate for the two cooling stages. This cooling rate is V=30-150 K/s and preferably V=50-90 K/s, depending on the geometry of the hot-rolled strip and the chemical composition of the steel grade that is used. In regard to these cooling rates, it should be noted that a cooling rate less than 30 K/s is not possible due to the small amount of time that is available in the conventional cooling line of a continuous casting and rolling plant, and that cooling rates greater than 150 K/s likewise cannot be realized in cooling lines of this type, which consist of a succession of water cooling zones spaced a certain distance apart.

The hot-rolled strip produced with the method of the invention with TRIP steel properties for different strength levels with an elastic limit tensile strength ratio R_(p0.2)/R_(m) in the range of 0.45-0.75 has the following combinations of tensile strength Rm and percentage elongation after fracture A:

R _(m)=600-700 MPa

A>25%

R _(m)=700-800 MPa

A>23%

R _(m)=800-900 MPa

A>21%

R _(m)=900-1,000 MPa

A>18%

R _(m)>1,000 MPa

A>15%

Further details and advantages of the invention are explained in greater detail below with reference to the specific embodiment of the invention illustrated in the accompanying drawings.

FIG. 1 shows a CSP plant.

FIG. 2 shows a modified cooling line of the CSP plant.

FIG. 3 shows cooling curves for a dual-phase steel and a TRIP steel in a TTT diagram.

FIG. 1 shows the layout of a conventional CSP plant 1 schematically. In the illustrated example, it comprises the following main components in the direction of conveyance (from left to right in the drawing): the casting installation with two strands 2, the strand guides 3, the soaking furnaces 4 with a furnace transverse conveyor 5, a multiple-stand rolling mill 6, the cooling line 10, and coilers 8.

FIG. 2 shows a modified cooling line 10 of a CSP plant 1, which is necessary for carrying out cooling in accordance with the invention and is already known from EP 1 108 072 B1,which describes a method for producing dual-phase steel. This modified cooling line 10 of the CSP plant 1 is installed downstream of the last finish rolling stand 6′. The cooling line 10 has several successive water cooling zones 11 ₁₋₇, 12 that are spaced a certain distance apart and can be automatically controlled. The water cooling zones 11 ₁₋₇, 12 are equipped with water spray heads 13, which evenly spray the upper and lower surfaces of the hot-rolled strip 7 with a specific amount of water. The positioning of the water cooling zones 11 ₁₋₇, 12 within the cooling line 10, their number, their spacing, and the number of water spray heads 13 per water cooling zone 11 ₁₋₇, 12 are chosen in such a way that the desired cooling rate of the two cooling stages can be variably adjusted in advance in order to achieve optimum adaptation of the water cooling zones 11 ₁₋₇, 12 to the cooling. conditions that are to be adjusted. Automatic control of the amount of water sprayed thus makes it possible, even during the cooling operation, to make any necessary change in the cooling rate.

An additional water cooling zone 12 is installed a greater distance from the last water cooling zone 117 of the first cooling stage than the distance between the individual zones of water cooling zones 11 ₁₋₇. The second cooling stage is carried out in this additional water cooling zone 12. In this water cooling zone 12, in contrast to the water cooling zones 11 ₁₋₇ of the first cooling stage, there is a significantly larger number of water spray heads 13 in order to carry out forced intensive cooling over a shorter distance. The distance between the last water cooling zone 11 ₇ of the first cooling stage and the water cooling zone 12 of the second cooling stage is chosen sufficiently large to obtain the holding time necessary to achieve transformation of the austenite to at least 40% ferrite, as prescribed by the invention, at the predetermined strip speed.

FIG. 3 shows a TTT diagram with the transformation lines for ferrite, pearlite, and bainite and with the temperature lines (20, 21, 22, 24) for Ac₃, Ac₁, and M_(S). Horizontal shift arrows 27 for the transformation lines and vertical shift arrows 28 for the temperature lines show the effect of existing or added alloying elements on the position of these transformation and temperature lines in the TTT diagram. The cooling curve 25 for the production of a dual-phase steel and the cooling curve 26 for the production of a TRIP steel in accordance with the invention are plotted in this TTT diagram as examples. At approximately the same start temperature (above Ac₃) at the start of cooling and approximately the same holding time temperature (above Ac₁), a significantly different microstructural composition is obtained due to the different courses of the cooling and the different compositions of the initial steels. According to the plotted cooling curve 25 for the dual-phase steel, the cooling curve 25 passes only into the ferrite range and ends below the martensite start temperature line 22, which is well above room temperature 23, so that, as desired, a dual microstructure that consists only of ferrite and martensite is obtained. On the other hand, the cooling curve 26 for the production of a TRIP steel in accordance with the invention passes first through the ferrite range and then through the bainite range and ends above the martensite start temperature line 24, which is now below room temperature 23, so that transformation to martensite during cooling does not take place, and, in accordance with the invention, a microstructure is obtained that consists of ferrite, bainite, and some retained austenite.

LIST OF REFERENCE NUMBERS

-   1 CSP plant -   2 casting installation with two strands -   3 strand guide -   4 soaking furnace -   5 furnace transverse conveyor -   6 multiple-stand rolling mill 6 -   6′ last rolling stand -   7 hot-rolled strip -   8 coiler -   9 temperature measurement -   10 cooling line -   11 water cooling zones -   12 water cooling zone -   13 water spray heads -   20 Ac₃ temperature line -   21 Ac₁ temperature line -   22 martensite start temperature line for a dual-phase steel -   23 room temperature line -   24 martensite start temperature line for a TRIP steel -   25 cooling curve for a dual-phase steel -   26 cooling curve for a TRIP steel -   27 horizontal shift arrows of the transformation lines -   28 vertical shift of the temperature lines 

1. A method for producing hot-rolled strip that consists of TRIP (transformation-induced plasticity) steel with both high strength values and outstanding deformation properties, a refinement of dual-phase steels with a predetermined chemical composition of the steel grade that is used within the following limits: 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements, and a multiphase microstructure which consists of 40-70% ferrite, 15-45% bainite, and 5-20% retained austenite, from the hot-rolled state in a thin-slab continuous casting and rolling plant (CSP plant) (1), wherein the finish rolling of the hot-rolled strip (7) for adjusting a very fine austenite grain (d<8 μm) during the last deformation is carried out at temperatures of 770-830°0 C., just above Ar₃ in the range of metastable austenite, and wherein immediately after the last rolling stand (6′), a controlled two-stage cooling of the hot-rolled strip (7) to a strip temperature in the range of bainite formation of 320-480° C. is carried out with a holding time at about 650-730° C., whose start is determined by the entry of the cooling curve (26) into the ferrite range and whose duration is determined by the transformation of the austenite to at least 40% ferrite, wherein the controlled two-stage cooling of the hot-rolled strip (7) is carried out in a cooling line (10) that consists of a succession of water cooling zones (11 ₁₋₇, 12) that are spaced a certain distance apart, can be automatically controlled, and are equipped with water spray heads (13), which evenly spray the upper and lower surfaces of the hot-rolled strip 7 with a specific amount of water.
 2. A method in accordance with claim 1, wherein the cooling rate is V=30-150 K/s, and preferably V=50-90 K/s, depending on the chemical composition of the steel grade that is used and on the geometry of the hot-rolled strip (7).
 3. A method in accordance with claim 1, wherein the controlled two-stage cooling of the hot-rolled strip (7) is carried out in a cooling line (10) that consists of a succession of water cooling zones (11 ₁₋₇, 12) spaced a certain distance apart.
 4. A hot-rolled strip (7) with TRIP steel properties, characterized by a chemical composition within the following limits: 0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and customary accompanying elements; an elastic limit tensile strength ratio R_(p0.2)/R_(m) in the range of 0.45-0.75; and a possible level of strength with respect to combinations of tensile strength R_(m) and elongation after fracture A: R _(m)=600-700 MPa

A>25% R _(m)=700-800 MPa

A>23% R _(m)=800-900 MPa

A>21% R _(m)=900-1,000 MPa

A>18% R _(m)>1,000 MPa

A>15%. 